Phased arrays of widely spaced antenna elements can be used to determine the direction of arrival of electromagnetic or acoustic signals. In such arrays, the spatial frequency signals are individually processed in the course of determining signal direction to form a single, scannable beam.
U.S. Pat. No. 4,213,131 discloses the basic techniques used in forming single beams using spatial frequencies. This prior patent teaches conversion of each antenna element signal to two or more different first intermediate frequency signals using coherent local oscillators. The intermediate frequency signals are mixed with intermediate frequency signals of other antenna elements to form second intermediate frequency signals. The patent further teaches mixing pairs of selected second intermediate frequency signals to produce third intermediate frequency signals representing a complete set of continuous spatial harmonics. A set of odd spatial harmonics is summed to produce the "single beam" output. The signal processing system disclosed in the prior patent did not eliminate responses to sidelobes and provided no formation of error pattern signals.
Sidelobes are a natural phenomenon associated with all collimating systems, e.g., phased arrays, reflectors, lenses, horns, i.e., antennas. Sidelobe responses introduce ambiguities and interference. In conventional antenna systems, where the spatial frequencies are summed prior to the detection process, a signal's carrier is effectively amplitude modulated in the antenna's output as the signal sweeps through the radiation pattern, i.e. through the ".theta." plane of the antenna.
Conventional phased array systems do not process the spatial frequencies prior to detection and summing, and sidelobe responses cannot readily be eliminated. Even in non-conventional phased array systems such as that described in U.S. Pat. No. 4,213,131, where the spatial frequencies are processed prior to detection and summing, sidelobe responses are not specifically eliminated.